In recent years, a major technical challenge presented to the petroleum refining industry has been the requirement to establish alternate processes for manufacturing high octane gasoline in view of the regulated requirement to eliminate lead additives as octane enhancers as well as the development of more efficient, higher compression ratio gasoline engines requiring higher octane fuel. To meet these requirements the industry has developed non-lead octane boosters and has reformulated high octane gasoline to incorporate an increased fraction of aromatics. While these and other approaches will fully meet the technical requirements of regulations requiring elimination of gasoline lead additives and allow the industry to meet the burgeoning market demand for high octane gasoline, the economic impact on the cost of gasoline is significant. Accordingly, workers in the field have intensified their effort to discover new processes to manufacture the gasoline products required by the market place.
Gasolines manufactured to contain a higher concentration of aromatics such as benzene, toluene and xylenes can adequately meet the octane requirements of the marketplace for a high octane fuel. Aromatics, particularly benzene, are commonly produced in refinery processes such as catalytic reforming which have been a part of the conventional refinery complex for many years. However, their substitution for the environmentally unsuitable lead octane enhancers is complicated by environmental problems of their own. Environmental and health related studies have raised serious questions regarding the human health effects of benzene. The findings suggest that exposure to high levels of benzene should be avoided with the result that benzene concentration in gasoline to enhance octane number is limited and controlled to a relatively low value. Alkylated aromatics, such as toluene and xylenes do not suffer under the same health effects liabilites as benzene and can be readily used for their octane enhancing properties.
When hydrocarbons boiling in the gasoline boiling range are reformed in the presence of a hydrogenation-dehydrogenation catalyst, a number of reactions take place which include dehydrogenation of naphthenes to form aromatics, dehydrocyclization of paraffins to form aromatics, isomerization reactions and hydrocracking reactions. It is well known that reforming conditions can be varied to favor the production of preferred products. However, when the reforming conditions are severe, coke formation in the catalyst occurs with consequent deactivation of the catalyst. Clearly, the composition of the charge to the reformer will influence the reforming conditions selected and the composition of the reformate produced. For instance, it is known that the production of benzene in the reforming process is favored when the charge contains a significant portion of benzene precursors such as hexanes. Typically, both n-hexane and isohexane are converted in the reformer, although only n-hexane has an unacceptably low octane number for consideration as part of the gasoline pool.
The treatment of a reformate with crystalline aluminosilcate zeolites is known in the art and has included both physical treatments such as selective adsorption, as well as chemical treatments such as selective conversion thereof. In U.S. Pat. No. 3,770,614 to Graven a process combination is described for upgrading naphtha boiling range hydrocarbons by a combination of catalytic reforming and selective conversion of paraffinic components to enhance yield of aromatic hydrocarbons by contact with crystalline aluminosilicate catalyst having particular conversion characteristics. In U.S. Pat. No. 3,649,520 to Graven a process is described for the production of lead free gasoline by an integrated process of reforming, aromatics recovery and isomerization including C.sub.6 hydrocarbons upgrading to higher octane product for blending. The foregoing patents are incorporated herein by reference in their entirety.
It is an object of the present invention to provide a process for the manufacture of high octane lead free gasoline containing a reduced amount of benzene.
Another object of the present invention is to provide a process for the manufacture of low benzene content gasoline by avoiding reforming the high octane benzene precursors in the C.sub.6 aliphatic hydrocarbon feedstream to a reformer.
Yet another object of the present invention is to provide a process for the manufacture of high octane gasoline containing alkylaromatics produced by the alkylation of benzene components in a reformate and employing acidic metallosilicate as alkylation catalyst.